<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let observer = new MutationObserver(() => console.log('body attributes changed'));

    observer.observe(document.body, { attributes: true });

    document.body.className = 'foo';
    console.log('changed body class');

    // changed body class
    // body attributes changed
  </script>
</body>
</html>